Pope Francis prays at Israel''s separation barrier after he made an unscheduled stop at the security wall drawing attention to the towering eight-metre high concrete wall topped by a guard tower.

Pope Francis has invited the Israeli and Palestinian presidents to come to the Vatican to pray for peace a month after US-backed talks aimed at ending the Middle East conflict collapsed.

"In this, the birthplace of the Prince of Peace, I wish to invite you, President Mahmoud Abbas, together with President Shimon Peres, to join me in heartfelt prayer to God for the gift of peace," the pope said at a Mass in Bethlehem.

"I offer my home in the Vatican as a place for this encounter of prayer," Francis said, in what Vatican spokesman Father Federico Lombardi described as an unprecedented papal initiative.

"All of us - especially those placed at the service of their respective peoples - have the duty to become instruments and artisans of peace, especially by our prayers," the pope said. "Building peace is difficult, but living without peace is a constant torment."

Asked about the invitation, a spokeswoman for Peres said in Jerusalem that he "always accepts any kind of initiative to promote peace". While Abbas heads the Palestinian government, Peres's presidential post is largely ceremonial.

Netanyahu broke off peace talks last month after Abbas signed a reconciliation deal with one of Israel's most bitter enemies, the Hamas Islamist group that runs the Gaza Strip.

"This is an open invitation," Lombardi said, adding that he hoped the visit could take place before Peres's presidential term ends in July.

The invitation to "men of good will", Lombardi said, "is one of the signs of the courage and creativity of Pope Francis in his efforts to bring about peace".

Pope Francis made a surprise stop at the hulking wall Palestinians see as a symbol of Israeli oppression today, minutes after begging both sides to end a conflict that he said was no longer acceptable.

In an image set to become one of the most emblematic of his trip to the holy land, a sombre-looking Francis rested his forehead against the concrete structure that separates Bethlehem from Jerusalem, and prayed silently as a child holding a Palestinian flag looked on.

He stood at a spot where someone had sprayed in red paint "Free Palestine". Above his head was graffiti in broken English reading: "Bethlehem look like Warsaw Ghetto", comparing the Palestinian plight with that of the Jews under the Nazis.

On the second leg of a three-day trip to the Middle East, Francis delighted his Palestinian hosts by referring to the "state of Palestine", giving support for their bid for full statehood recognition in the face of a paralysed peace process.

But, speaking at the birthplace of Jesus in the Palestinian-run city of Bethlehem in the Israeli-occupied West Bank, he made clear that a negotiated accord was needed, calling on leaders from both sides to overcome their myriad divisions.

TEL AVIV

From Bethlehem, where the Pope also visited a Palestinian refugee camp, he flew by helicopter to Tel Aviv airport where he was welcomed by Peres and Netanyahu, before flying back over the Judean hills to Jerusalem.

In a speech at the ceremony, Francis invoked "the right of the State of Israel to exist and to flourish in peace and security within internationally recognised boundaries".

At the same time, he said there must be "recognition of the right of the Palestinian people to a sovereign homeland and their right to live with dignity and with freedom of movement".

The Pope also recalled the Holocaust, using the Hebrew word for the term, and said that "ever mindful of the past" there can be "no place for anti-Semitism".

"A particularly moving part of my stay will be my visit to the Yad Vashem Memorial to the six million Jews who were victims of the Shoah," the Argentinian pontiff said. "I beg God that there will never be another such crime, which also counted among its victims many Christians and others."

Peres, welcoming the Pope in blustery, warm sunshine, said: "We are grateful to you for assuming your sensitive and resolute stand against all expressions of anti-Semitism, against all manifestations of racism."